The present invention relates to a presensitized plate for use in making a lithographic printing plate (hereunder referred to as "PS plate") and more specifically to a PS plate provided with a light-sensitive layer comprising an alkaline water-soluble or swellable photocrosslinkable polymer, in which the adhesion of the light-sensitive layer to a substrate having a hydrophilic surface is improved.
There have been well-known photocrosslinkable materials capable of causing crosslinking through a cyclization reaction and these materials have widely been employed as a principal ingredient of a light-sensitive composition for preparing PS plates or the like. As such photocrosslinkable polymers, useful are, for instance, polymers carrying maleimido groups on the side chains and polymers carrying, on the side chains or in the main chain, groups having a photodimerizable unsaturated double bond adjacent to an aromatic nucleus such as cinnamyl group, cinnamoyl group, cinnamylidene group, cinnamylideneacetyl group and chalcone group and some of these have already been put into practical use. In particular, polymers having, on the side chains, maleimido groups and polyester resins which have, in the molecular chain, cinnamic acid skeleton and prepared by condensing phenylenediacrylic acid or an alkyl ester thereof with glycol have relatively high sensitivity.
In addition, it is preferable to use a developer free of an organic solvent from the viewpoint of safety of working environment. For this reason, it has been tried to make these polymers alkaline water-soluble. Examples of such polymers include copolymers of N-[2-(methacryloyloxy)ethyl) -2,3-dimethylmaleimide with methacrylic acid or acrylic acid as disclosed in Die Angewandte Makromolekulare Chemie, 1984, 128 ; and polymers having, on the side chains, photodimerizable functional groups and carboxyl groups as disclosed in Japanese Patent Unexamined Publication (hereunder referred to as "J.P. KOKAI") Nos. Sho 62-175729, Sho 62-175730, Sho 63-25443, Sho 63-218944 and Sho 63-218945.
However, these polymers do not have sufficient adhesion to an aluminum substrate and, therefore, they cannot be widely used for preparing light-sensitive layers of PS plates. In other words, they are greatly limited in applications. Thus, these light-sensitive polymers have not widely been used as a material for the light-sensitive layer of PS plates. If the adhesion of the light-sensitive layer to the substrate is low, PS plates which can provide lithographic printing plates having desired durability cannot be obtained. More specifically, images are liable to be peeled off when the plate is rubbed with a brush or the like during development and during printing operation. In particular, this tendency becomes conspicuous when the plate is lightly exposed to light and as a result, the sensitivity is impaired.
Some attempts have been made to eliminate this problem of low adhesion to a substrate. For instance, Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") No. Sho 46-26521 discloses a substrate obtained by applying an anodized layer onto an aluminum substrate and then electrolyzing the layer in phosphoric acid electrolyte; J.P. KOKAI Nos. Sho 49-8428, Sho 49-12903 and Sho 50-138903 disclose aluminum substrates obtained by electrolyzing an aluminum plate in sulfuric acid electrolyte and etching the plate with phosphoric acid or polyphosphoric acid; and J.P. KOKAI No. Sho 49-93101 discloses the use of an aluminum substrate obtained by electrolyzing an aluminum plate in sulfuric acid electrolyte and then etching the plate with an alkaline solution. Alternatively, J.P. KOKOKU No. 50-7481(Brit. Pat. 1274017) and J.P. KOKAI No. Sho 62-78544(U.S. Pat. No. 4,845,009) disclose methods in which an underlying coating of a negative-working diazo resin is applied onto the surface of an aluminum substrate or a negative-working diazo resin is added to the light-sensitive layer.
However, in the techniques in which the adhesion of a light-sensitive layer to an oxidized layer of a substrate is enhanced according to a variety of treating methods, the adhesion of the oxidized layer to the light-sensitive layer becomes excessively high and the resulting non-image areas are liable to receive ink during printing and thus the resulting lithographic printing plate often causes background contamination. Moreover, in the foregoing techniques in which an underlying coating of a negative-working diazo resin is applied onto the surface of an aluminum substrate or a negative-working diazo resin is added to the light-sensitive layer, the diazo resin remains unremoved on the non-image areas even after the development since the resin is substantially insoluble in an alkaline developer. This likewise becomes a cause of background contamination during printing operation.